JUNO detector filled and operational for neutrino research

JUNO detector filled and operational for neutrino research
by Riko Seibo
Tokyo, Japan (SPX) Sep 01, 2025

The Jiangmen Underground Neutrino Observatory (JUNO) in Guangdong Province has completed the filling of its 20,000 tons of liquid scintillator and initiated data taking, marking a major milestone after more than a decade of construction. The experiment is the first of a new generation of large-scale neutrino detectors to become fully operational. Early commissioning results confirm that the detector exceeds design specifications, positioning JUNO to resolve the ordering of neutrino masses, a central question in particle physics.

Situated 700 meters underground, JUNO measures antineutrinos from nearby nuclear power plants with record precision. Its approach is largely unaffected by Earth matter effects, allowing clean determination of neutrino mass ordering. JUNO will also refine measurements of neutrino-oscillation parameters and study neutrinos from the Sun, supernovae, and the atmosphere, while probing exotic physics such as sterile neutrinos and proton decay.

Construction began in 2015 after approval by the Chinese Academy of Sciences and Guangdong Province in 2013. Detector assembly was completed in late 2024, followed by a carefully staged filling campaign that maintained ultra-high purity, optical transparency, and structural safety. The detector's 35.4-meter acrylic sphere holds the scintillator at the center of a 44-meter-deep water pool, supported by a stainless steel truss and monitored by 20,000 large and 25,600 small photomultiplier tubes.

JUNO spokesperson Prof. Yifang Wang of IHEP stated, "Completing the filling of the JUNO detector and starting data taking marks a historic milestone. For the first time, we have in operation a detector of this scale and precision dedicated to neutrinos. JUNO will allow us to answer fundamental questions about the nature of matter and the universe."

Chief Engineer Prof. Xiaoyan Ma noted that building JUNO required years of planning, new technologies, and rigorous safety and purity standards, crediting the success to the dedication of hundreds of engineers and technicians.

More than 700 scientists from 74 institutions across 17 countries participate in JUNO. German groups, supported by the DFG and part of the PRISMA+ Cluster of Excellence at Johannes Gutenberg University Mainz, contributed through the OSIRIS pre-detector, sensitivity studies, and data analysis.

Prof. Livia Ludhova, a JUNO Executive Committee member, said: "JUNO is the result of many years of international collaboration. Our teams have contributed important building blocks to the current success... It is very satisfying to see how our combined expertise has now come together in a detector that will serve the global physics community for decades to come."

Designed for up to 30 years of operation, JUNO also has a potential upgrade path to search for neutrinoless double-beta decay, a pursuit that could determine the absolute neutrino mass scale and whether neutrinos are Majorana particles. Such discoveries would profoundly advance particle physics, astrophysics, and cosmology.